Braid-reinforced hollow fiber membrane

ABSTRACT

Disclosed herein is a braid-reinforced hollow fiber membrane, comprising an active layer formed on the surface of a tubular braid, wherein the braid is made of a mixture of metal wire with polymer fiber. The hollow fiber membrane can be used in separation processes at high pressure due to the pressure resistance thereof, can be used in the separation of organic solvents at high temperature due to the dimensional stability thereof, and in addition, can be used in various applications due to the electrical conductivity thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a braid-reinforced hollow fiber membrane, and more particularly to a metal wire-containing braid-reinforced hollow fiber membrane, which is manufactured by making a tubular braid using a mixture of metal wire with polymer fiber and forming an active layer on the outer surface of the tubular braid, and thus can be used even in high-pressure liquids or high-temperature organic solvents.

2. Background of the Related Art

In order to use hollow fiber membranes under high pressure, the mechanical stability of the membranes against external pressure must be ensured, and in order to use the hollow fiber membranes in high-temperature organic solvents, the dimensional stability of the membranes is required. In order to improve the stability of the membrane against external pressure, the thickness of the membrane may be increased and the porosity of the membrane may be reduced. This causes a new problem in that the permeation rate of liquid through the membrane becomes slow. For this reason, various attempts have been made to reinforce the active layer of the hollow fiber membrane with a tubular braid.

In the prior art, hollow fiber membranes, which are used mainly in membrane separation processes for treating non-solvent solutions at room temperature, including wastewater treatment processes and biological membrane processes, were manufactured by making a tubular braid only using polymer fiber and forming an active layer on the outer layer of the tubular braid. Examples of the polymer fiber used to make the braid generally include polyester, nylon, aramid, polypropylene and polyethylene fibers, and among them, a suitable polymer fiber material is selected according to the intended use thereof.

To make the braid of the hollow fiber membrane which is used in membrane separation processes at high temperature, glass fiber is also used. The thickness and inner diameter of the braid are determined depending on the kind of fiber used to make the braid, the thickness (denier) of each filament of yarn, the number of the filaments and the number of strands of yarn which is used to make the braid.

The above-described polymer fiber braid-reinforced hollow fiber membranes have very excellent resistance against pressure in the lengthwise direction thereof, that is, have very excellent tensile strength. However, there are problems in that they have low resistance against pressure in the direction perpendicular to the membrane surface, that is, pressure in the radial direction, and thus if the pressure in the radial direction is increased, the membranes are likely to lose their original shape. For example, where the polymer fiber braid-reinforced hollow fiber membranes are used in reverse osmosis membrane separation processes at a pressure higher than 20 atm, the active layer of the membranes does not maintain the circular tubular shape, and the cross-sectional shape thereof is collapsed by the pressure.

In general, the active layer made of polymer material does not have dimensional stability in high-temperature organic solvents. In the case of the above-described prior polymeric fiber braid-reinforced hollow fiber membranes, not only the dimension of the active layer but also the dimension of the braid itself changes in organic solvent solutions at high temperature. If the dimensional stability is lost as described above, there is a problem in that significant stress occurs at the interface between the braid and the active layer, thus causing damage to the active layer.

Accordingly, the prior polymeric fiber braid-reinforced hollow fiber membranes are not problematic when they are used in non-solvent liquid at low pressure and room temperature, but they are difficult to use in high-pressure liquid or high-temperature organic solvent liquid.

Accordingly, the braid needs to be improved for the shape stability or dimensional stability of the hollow fiber membranes, because the active layer is applied on the braid used as a reinforcing material, and the braid mainly copes not only with mechanical loads, but also dimensional changes resulting from exposure to high-temperature organic solvents.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve the problems with the prior polymeric fiber braid-reinforced hollow fiber membranes, and it is an object of the present invention to provide a braid-reinforced hollow fiber membrane, the shape of which can be maintained, because it has high pressure resistance even in high-pressure liquid, and the dimensional stability of which can be ensured, because the dimension thereof does not change even in high-temperature organic solvents.

Another object of the present invention is to provide a braid-reinforced hollow fiber membrane, which has electrical conductivity, and thus can be used in various applications.

To achieve the above objects, the present invention provides a hollow fiber membrane, comprising an active layer formed on the surface of a tubular braid, wherein the braid is made of a mixture of metal wire with polymer fiber.

The metal wire is preferably one or more selected from among copper wire, nickel wire, stainless steel wire, tin wire and nichrome wire. Also, the metal wire preferably has a diameter of 0.05-0.40 mm.

As described above, hollow fiber membranes to be used in high-pressure liquids particularly require mechanical stability, and hollow fiber membranes to be used in high-temperature organic solvents particularly require dimensional stability. The physical properties of the braid need to be improved, because it is difficult to ensure the mechanical strength or dimensional stability of the active layer itself, and the active layer reinforced only with a polymer fiber braid is unstable in conditions of high temperature, high pressure and contact with organic solvents. In this viewpoint, metal wires have significantly excellent mechanical properties compared to those of polymer fibers and, in addition, have very high dimensional stability, because they are heat-resistant and have no affinity for organic solvents upon contact with the organic solvents. When part of the polymer fiber of the braid is substituted with metal wire, the stability of the braid against pressure and temperature and contact with organic solvent are determined by the metal wire which is the major reinforcing material. As a result, the braid made of a mixture of metal wire with polymer fiber is very stable even when it is brought into contact with high-pressure liquids or high-temperature organic solvents, and thus a membrane reinforced with this braid will also necessarily be stable.

In the case where a braid is made using a mixture of metal wire with polymer fiber, as the ratio of the metal wire in the braid is increased, the mechanical properties and dimensional stability of the braid become excellent, but the braid itself becomes hard, and thus the handling property thereof becomes poor, when it is applied with an active layer or is manufactured into modules. On the other hand, if the ratio of the metal wire is reduced, the flexibility of the braid is increased to improve the the handling property thereof, but the mechanical properties and dimensional stability thereof are deteriorated. Thus, the number of strands of the metal wire in the braid is preferably 5-30% of the total number of strands of braided yarn in the braid. In addition to the case where the mixture of metal wire with polymer fiber is wound around bobbins, only the metal wire may, if necessary, be wound around bobbins to make a braid. When only the metal wire is wound around bobbins, the thickness of the metal wire must be larger than that of the metal wire in the mixture with the polymer fiber. As the metal wire, either a single metal wire or a metal wire assembly consisting of 2-5 thin metal wires is used.

The polymer fiber is preferably selected from among polyester, nylon, aramid, polypropylene and polyethylene fibers. The polymer fiber may also be yarn consisting of 10-400 single filaments having a thickness of 100-500 denier. To make the braid, yarn (polymer fiber and metal wire) consisting of 10-80 strands is preferably used.

On the outer surface of the above-described inventive braid, a microfiltration membrane or an ultrafiltration membrane may be formed as an active layer. The microfiltration membrane or the ultrafiltration membrane is formed to a given thickness by either dissolving a solvent, selected from polysulfone, polyethersulfone, polyimide, polyetherimide, polyacrylonitrile, polyethylene terephthalate, polybutylene terephthalate, nylon, polyphenyl sulfite, polyethylene and polypropylene, in a solvent, or mixing the selected polymer with a diluent at high temperature, to make a uniform solution, applying the solution on the braid, and then treating the applied solution using known diffusion-induced phase separation or thermal-induced phase separation.

On the microfiltration membrane or the ultrafiltration membrane, a nanofiltration membrane, a reverse osmosis membrane, a vapor permeation membrane, a pervaporation membrane or a gas separation membrane may additionally be formed in the form of a composite membrane applied with a thin active layer.

The hollow fiber membrane produced as described above maintains excellent shape stability and dimensional stability, even when it is used in high-pressure liquids or high-temperature organic solvents.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which:

FIG. 1 a is a partially sectioned perspective view of a braid-reinforced hollow fiber membrane according to a first embodiment of the present invention;

FIG. 1 b is a cross-sectional view of the braid-reinforced hollow fiber membrane shown in FIG. 1 a;

FIG. 1 c shows the arrangement of metal wire bobbins and polymer fiber bobbins in the braid shown in FIG. 1 a;

FIG. 2 a is a partially sectioned perspective view of a braid-reinforced hollow fiber membrane according to a second embodiment of the present invention;

FIG. 2 b is a cross-sectional view of the braid-reinforced hollow fiber membrane shown in FIG. 2 a; and

FIG. 2 c shows the arrangement of metal wire bobbins and polymer fiber bobbins in the braid shown in FIG. 2 a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, preferred embodiments of a braid-reinforced hollow fiber membrane according to the present invention will be described in detail with reference to the accompanying drawings.

The first embodiment shown in FIGS. 1 a and 1 b is a braid-reinforced hollow fiber membrane, manufactured by mixing 2 metal wire strands 3 with 34 polymer fiber strands 5 to make a braid and then forming an active layer 1 on the outer surface of the braid. The second embodiment shown in FIGS. 2 a and 2 b is a braid-reinforced hollow fiber membrane, manufactured by mixing 4 metal wire strands 3 with 32 polymer fiber strands to make a braid and then forming the active layer 1 on the outer surface of the braid.

Referring to FIGS. 1 a and 1 b or FIGS. 2 a and 2 b, the hollow fiber membrane according to the present invention is characterized in that part of the polymer fiber 5 in the braid is substituted with the metal wire 3 in order for the hollow fiber membrane to have pressure resistance even in high-pressure liquids and in order for the dimension thereof not to change even in high-temperature organic solvents.

Although FIGS. 1 a and 1 b or FIGS. 2 a and 2 b illustratively show that the single strand of the polymer fiber consists of a single filament and that the single strand of the metal wire consists of a single metal wire, it is possible to use yarn as the polymer fiber as described above and to use a metal wire assembly as the metal wire.

Referring to FIG. 1 c or 2 c, the thickness of the braid is determined by the thickness of the polymer fiber strands and the thickness of the metal wire strands. Also, in order to make the braid having uniform thickness, the thickness of the polymer fiber strands is substantially equal to the thickness of the polymer fiber strands.

As described above, the braided yarn (polymer fiber and metal wire) consisting of 10-80 strands is used to make the braid. For this purpose, the polymer yarn and the metal wire, which consist of 10-80 strands, are wound around bobbins, and then, as shown in FIGS. 1 c and 2 c, the bobbins are mounted in a ring-shaped carrier 11 of a braiding machine. Herein, the number of polymer fiber bobbins 9 mounted in the carrier and the number of strands of the polymer fiber in the braid are equal to each other, and the number of metal wire bobbins 7 and the number of strands of the metal wire in the braid are equal to each other. Thus, the ratio of the metal wire in the braid is expressed as the percentage of the number of the metal wire bobbins relative to the total number of the bobbins. For example, when the total number of the bobbins is 36 and the number of the metal wire bobbins is 3, the ratio of the metal wire is 8.33%. As described above, in order for the ratio of the number of strands of the metal wire relative to the total number of strands of braided yarn in the braid to be maintained at 5-30%, the number of the metal wire bobbins (including metal wire strands combined with polymer fiber) mounted in the braiding machine carrier 11 must also be maintained at 5-30% relative to the total bobbin number.

Each of the metal wires in the braid is preferably braided while maintaining a constant distance from the adjacent metal wires. For this purpose, as shown in FIG. 1 c or 2 c, the metal wire bobbins are arranged symmetrically with respect to the center of the ring-shaped carrier. If the metal wire bobbins are not arranged symmetrically with respect to the center of the ring-shaped carrier, the surface of the resulting braid becomes non-smooth and non-uniform.

On the surface of the braid made using the mixture of the metal wire and the polymer fiber at a given ratio, the active layer can be formed to manufacture a braid-reinforced hollow fiber membrane. Where a microfiltration membrane or an ultrafiltration membrane is formed as the active layer, a porous polymer membrane is formed to a given thickness by either dissolving a solvent, selected from polysulfone, polyethersulfone, polyimide, polyetherimide, polyacrylonitrile, polyethylene terephthalate, polybutylene terephthalate, nylon, polyphenyl sulfite, polyethylene and polypropylene, in a solvent, or mixing the selected polymer with a diluent at high temperature, to make a uniform solution, applying the solution on the braid, and then treating the applied solution using known phase separation techniques, such as diffusion-induced phase separation or thermal-induced phase separation.

On the microfiltration membrane or the ultrafiltration membrane, a nanofiltration membrane, a reverse osmosis membrane, a vapor permeation membrane, a pervaporation membrane or a gas separation membrane may further be formed in the form of a composite membrane applied with a thin active layer.

Hereinafter, the present invention will be described in further detail with reference to examples, and the properties of braid-reinforced hollow fiber membranes of the present invention will be compared with those of prior braid-reinforced hollow fiber membranes.

EXAMPLE 1

34 strands of 300/150 polyester and 2 strands of 0.2 mm-diameter stainless steel wire were mixed and braided, and the braid had an outer diameter of 2 mm and an inner diameter of 1 mm. 500 g of polysulfone and 130 g of polyvinylpyrrolidone were dissolved in 1370 g of dimethyl acetamide to prepare a uniform solution. The solution was uniformly applied on the outer surface of the braid including the metal wire, and was then cured in water, thus manufacturing a braid-reinforced hollow membrane having a porous active layer.

EXAMPLE 2

32 strands of 300/150 polyester yarn and 4 strands of 0.2 mm-diameter stainless wire were mixed and braided, and the braid had an outer diameter of 2 mm and an inner diameter of 1 mm. 500 g of polysulfone and 130 g of polyvinylpyrrolidone were dissolved in 1370 g of dimethyl acetamide to prepare a uniform solution. The solution was uniformly applied on the outer surface of the braid including the metal wire, and was then cured in water, thus manufacturing a braid-reinforced hollow fiber membrane having a porous active layer.

COMPARATIVE EXAMPLE

36 strands of 300/150 polyester yarn were braided, and the braid had an outer diameter of 2 mm and an inner diameter of 1 mm. 500 g of polysulfone and 130 g of polyvinylpyrrolidone were dissolved in 1370 g of dimethyl acetamide to prepare a uniform solution. The solution was uniformly applied on the outer surface of the polyester braid, and then cured in water, thus a braid-reinforced hollow fiber membrane having a porous active layer.

The length of each of the formed active layers was measured after drying, and then each of the hollow fiber membranes was left to stand in ethanol and n-hexane at 50° C. for 24 hours. Then, the length of each of the membranes was measured, thus estimating the dimensional stability thereof. Also, a module was manufactured using each of the manufactured membranes, water having pressure of 10, 20 or 40 atm was applied to the hollow fiber membrane mounted in the module, and the change in the cross-section of the hollow fiber membrane in the module was observed.

TABLE 1 Test results for dimensional stability and pressure stability of metal wire-containing braid-reinforced hollow fiber membranes according to the present invention and polymer fiber braid-reinforced hollow fiber membrane Comparative Items Example Example 1 Example 2 Dimensional Ethanol 2.74% 0.20% 0.09% stability¹ n-hexane 4.23% 0.21% 0.12% Pressure stability² 10 atm Stable Stable Stable 20 atm Collapsed Stable Stable 40 atm Collapsed Stable Stable ¹% = (swollen length-dried length)/dried length × 100 ²observed shape of cross-section of hollow fiber membrane under applied pressure

The hollow fiber membrane reinforced with the braid containing no metal wire (Comparative Example 1) showed an increase in length of 2.5-4.3% due to swelling in the organic solvents, and was observed to be collapsed when a pressure higher than 20 atm was applied thereto. However, the metal wire-containing braid-reinforced hollow fiber membranes according to Examples 1 and 2 of the present invention showed excellent dimensional stability and also were observed to maintain a stable membrane shape even at a pressure up to 40 atm. These results suggest that the metal wire included in the braid improves the dimensional stability of the membrane in the organic solvents and increases the shape stability of the membrane against pressure.

As described above, the hollow fiber membrane according to the present invention can be used in separation processes at high pressure due to the pressure resistance thereof, can be used in the separation of organic solvents at high temperature due to the dimensional stability thereof, and in addition, can be used in various applications due to the electrical conductivity thereof.

Although the present invention has been described for illustrative purposes with reference to the accompanying drawings and embodiments, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

1-10. (canceled)
 11. A hollow fiber membrane, comprising: a hollow, tubular braid of a number of spiraled, bare, metal wires and a number of spiraled polymer fibers, wherein the metal wires are spaced from each other with the polymer fibers therebetween; and an active layer formed on an outer surface of the tubular braid, wherein a ratio of the number of metal wires relative to a number of total wires and fibers in the braid is 5-30%, and wherein the metal wires provide electrical conductivity to the membrane.
 12. The membrane of claim 11, wherein the metal wires are selected from the group consisting of: copper wire, nickel wire, stainless steel wire, tin wire, and nichrome wire.
 13. The membrane of claim 11, wherein the metal wires each have a diameter of 0.05-0.40 mm.
 14. The membrane of claim 11, wherein the polymer fibers are selected from the group consisting of: polyester fiber, nylon fiber, aramid fiber, polypropylene fiber, and polyethylene fiber.
 15. The membrane of claim 11, wherein a total number of the polymer fibers and metal wires is 10-80. 